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Customer’s Engine Stumbles In Acceleration: What To Test

FEBRUARY 3, 2014 By Powertrain Pro

Ideally, an engine will accelerate smoothly through its power band. However, there are problems that can cause it to stumble (you might be more familiar with the terms judder, hesitate or hiccup). There are quite a few problems that can cause stumbling during engine operation, but you’ll have to do a few tests to find out exactly what’s going on. You’ll also have to make sure that the problem is actually stumbling and not missing or backfiring (test drive the car yourself to duplicate your customer’s concern).

Reasons an Engine Stumbles in Acceleration

One of the most common causes of stumbling or hesitation during acceleration is a fouled spark plug (or plugs). This is a simple diagnosis and a simple fix. Remove the plugs and check the electrodes for wear and/or oil fouling them. If the spark plugs are worn out, replace them. If they’re fouled, clean them off and replace them, then test the engine acceleration. If the problem doesn’t lie with the spark plugs, though, you’ll need to check other components especially when looking at engines for sale.

The airflow sensor or airflow meter is another common culprit here. Dead spots on this sensor can cause issues with combustion during acceleration, so you’ll need to troubleshoot the sensor for proper operation (electrical diagnostic). Another potential issue is a vacuum leak. Large leaks will cause issues on acceleration (usually as soon as you begin accelerating). The best way to test for a vacuum leak is to pressurize the system (with the engine off). Connect compressed air to the system and listen for leaks. This will let you pinpoint the problem area and make repairs. Vacuum leaks are usually very simple fixes (gaskets and hoses for the most part), but they can be very hard to spot, so be prepared to spend some time here.

If the problem isn’t actually stumbling/hesitation and instead involves missing or backfiring, you’ll need to take additional steps. Missing during acceleration can be an indication of spark plug or spark plug wire failure, a fuel filter that needs to be replaced or a vacuum leak. Backfiring is often a problem with the ignition system or a vacuum leak, but it can also be a problem with the vehicle’s computer.

Stumbling during acceleration is generally an easy fix once you have identified the problem. For your customer’s sake, it’s hopefully nothing more serious than fouled plugs or an easily identifiable vacuum leak.